The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrophotographic imaging process (U.S. Pat. No. 2,297,691) involves placing a uniform electrostatic charge on a photoconductive insulating layer known as a photoconductor or photoreceptor, exposing the photoreceptor to a light and shadow image to dissipate the charge on the areas of the photoreceptor exposed to the light, and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic toner material. The toner will normally be attracted to those areas of the photoreceptor which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This developed image may then be transferred to a substrate such as paper. The transferred image subsequently may be permanently affixed to the substrate by heat, pressure, a combination of heat and pressure, or other suitable fixing means such as solvent or overcoating treatment.
Also well known are techniques to develop such electrostatic images. Developer is a vehicle in which are dispersed charged colored toner particles. The photoreceptor bearing the electrostatic latent image is contacted with the developer. The contact causes the charged toner particles in the developer to migrate to the charged areas of the photoreceptor to develop the latent image. Then, the photoreceptor with the charged colored particles adhering to the latent image in image configuration. The developed image is then typically transferred to a suitable substrate, such as paper or transparency material, and optionally may be fixed to the substrate by heat, pressure or other suitable means.
Toners and developer compositions including colored particles are well known. Some U.S. Pat. Nos. in this regard are 5,352,521; 4,778,742; 5,470,687; 5,500,321;,5,102,761; 4,645,727; 5,437,953; 5,296,325; and 5,200,290. The traditional compositions normally contain toner particles consisting of resin and colorants, wax or a polyolefin, charge control agents, flow agents and other additives. A typical toner formulation generally contains about 90-95 weight percent resin, about 2-10 weight percent colorant, 0-about 6 weight percent wax, 0-about 3 weight percent charge control agent, about 0.25-1 weight percent flow agent and 0-about 1 weight percent other additives. Major resins are styrene-acrylic copolymers, styrene-butadiene copolymers and polyesters. The colorants usually are selected from cyan dyes or pigments, magenta dyes or pigments, yellow dyes or pigments, black dyes or pigments, and mixtures thereof.
One of the main advantages of selecting organic dyes instead of pigments for color toner compositions resides in the provisions of increased color fidelity as the dyes can be molecularly dispersed in the toner resins. To obtain a homogeneous dispersion, it is generally necessary to build into these molecules certain substituents for enhancing their compatibility with the toner resin. Unless the dye molecules are substantially fully compatible with the toner resins, they have a tendency to aggregate with time, especially when subjected to heat, pressure and humidity thereby resulting in a loss of color fidelity. Additionally, the low molecular weight of the dye molecules causes a high lability or mobility of the dye molecules in the toner resin resulting in undesirable bleeding of the dyes.
Conventional color toners are produced by a milling process described, for example, in the afore-mentioned U.S. Pat. No. 5,102,761. In that process, a polyester resin is compounded with pigments, charge control agents ("CCA") and occasionally wax in a melt mixer. The resulting polymer mixture is mechanically crushed and then milled into small particles. The conventional toner particles typically have an irregular shape and a broad distribution in particle size. For optimum resolution of images and color, smaller particles perform better. Thus, for example, it is difficult to obtain resolutions better than about 400 dots/inch when the average particle size is more than about 7 .mu.m. For resolutions in the order of about 1200 dots/inch, particle sizes smaller than 5 .mu.m are typically needed. It is difficult to make particles smaller than about 7-10 .mu.m by conventional processes because of the high energy cost of producing small particles as well as uniform narrow particle size distribution.
Improvements to cure such efficiencies have been attempted in the past. For example, the afore-mentioned U.S. Pat. No. 5,352,521, 5,470,687 and 5,500,321 disclose toner particles produced by dispersion polymerization. In such method, monomers (typically styrenic and acrylate monomers) and additives such as pigments, CCA and wax are mixed together to form a dispersion. This is then further dispersed into an aqueous or a non-aqueous medium and the monomers are reacted to form toner particles. These particles, however, are deficient in uniform distribution of colorants, the transparency of the images as well as having a high cost. Furthermore, these processes are not useful to prepare polyester-based toner particles which are preferred over the styrenics or the acrylics due to their superior compatibility with pigments.
Pending applications 08/923,391, now U.S. Pat. No. 5,843,609, and 08/923,394 disclose polyesters and polyester-based toner particles which contain dye moieties in the backbone of main chain polyester repeat units.
There is continuing interest in the development of new and improved toner compositions for application in electrophotography.
Accordingly, it is an object of this invention to provide a polyester-based toner composition which has a superior combination of properties for electrophotographic imaging systems.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.